cook the brussels sprouts first, for the full 30 minutes.
you then could leave them in and lower the temp for your steaks, or move them to the fridge and add them back during the last 20 minutes of the steaks cook.
leaving them in may alter the texture, but i suspect that it wont alter significantly. however, i have never tried.
fridging them and adding them back later is probably the better bet, and i'd give them a quick blast on some cast iron and butter too.